Process innovation increasingly contributes to improvements in the temperature capability and component reliability of gas turbine engine components. Innovations in investment casting have produced: complex, thin-walled, air-cooled gas turbine engine blades; integrally cast rotors and nozzles; high temperature, creep-resistant, directionally solidified (DS) columnar grained and single crystal airfoils; hot isostatic pressing (HIP) of castings to densify casting shrinkage porosity; and proprietary techniques to form very fine grain castings. Advanced powder metal manufacturing and consolidation processing, coupled with advanced extrusion and forging processes, have provided the capability to produce fine grain disks which exhibit improved low-cycle fatigue strength.
Low pressure plasma spray technology has introduced a new method to produce fine grain components and coatings. Few process methods, however, have been developed which successfully combine the high temperature creep properties of large-grained structures with the tensile and low-cycle fatigue capabilities of fine-grained structures in a single component. It is the objective of this invention to provide such a unique processing capability. The following discussion describes processes utilized in the prior art.
Integrally cast rotors having an equiaxed microstructure have been successfully used in many small gas turbine applications. The need for increased thrust and horsepower in military and commercial aircraft has led to more demanding requirements. Consequently, designers have used the traditional separately bladed approach, i.e., fabricating a fine-grained, forged disk; machining slots in the disk to accept machined blade roots; and inserting cast blades of the desired grain structure into the slots, thereby achieving a mechanical attachment. Machining slots and blade roots are costly processing steps. This method also limits the number of blades that can be attached, especially in smaller engines. A design with a large number of blades is desirable for higher performance.
Turbine disks are fabricated by wrought processes which utilize either ingot or powder metal starting stock. The powder metal disks are generally consolidated by hot isostatic processing (HIP) and demonstrate reduced alloy segregation compared to ingot metallurgy. Powder metal disks are, however, susceptible to thermally induced porosity (TIP) from residual argon used in powder atomization. Any oxygen contamination of powders can form an oxide network resulting in metallographically detectable prior particle boundaries which are known sites of fracture initiation. These limitations make powder metal disks costly in terms of both processing and quality controls.
Those skilled in the art of turbine engine design have recognized the potential advantages of combining the ease of fabrication and the structural integrity of monolithic integrally cast rotors with the high performance capability obtainable in separately bladed turbine engine rotors. Several approaches have been developed to produce such a turbine rotor.
One approach involves casting an equiaxed, hollow blade ring and then diffusion bonding a separately produced powder metal disk to the inside diameter of the ring. Interference fit and brazing are usually required to achieve complete bonding during HIP'ing. This approach has the disadvantage of requiring four separate processes: 1) casting; 2) precision machining; 3) powder metal HIP consolidation; and 4) a second HIP operation to achieve final bonding. Each of these processes are expensive and may create additional costs arising from defect scrap losses.
Another method uses powder metal in an investment mold which has directionally solidified or single crystal cast blades within it. The mold is loaded in a metal can, covered with an inert pressure-transmitting media, vacuum sealed, and HIP'ed. This combined blade/powder metal approach has less process steps than the interference fit approach but is severely limited in dimensional control due to blade/mold movement during consolidation of the 65-70% dense powder.
In view of the above-described disadvantages of the known methods for fabricating turbine components, a method for forming such components which overcomes many of these limitations is disclosed.
Accordingly, it is an object of the invention to provide a method of making turbine rotors which combines the property advantages of fine-grained disks and conventional investment castings.
It is a further objective of the invention to provide a method of making a composite article comprised of different metal portions. The different metal portions can be of different compositions or may be of the same composition with different microstructures.
Additional objects and advantages will be set forth in part of the description which follows, and in part, will be obvious from the description, or may be learned by practice of the invention.